Defines HTML on the top of the sidebar.

<div class="definition">
    String, default = ''
</div>

If you set <span class="argument">topHTML</span> property it will appear on the top of the sidebar. The property will accept any HTML or
text string and will stay on top even if sidebar has a scroller.
<div style="height: 10px;"></div>

You can define it during object creation:
<textarea class="javascript">
let sidebar = new w2sidebar({
    name : 'sidebar',
    topHTML: 'some text',
    nodes: [
        { id: 'id-1', text: 'Level 1' },
        { id: 'id-2', text: 'Level 2' },
        { id: 'id-3', text: 'Level 3' }
    ]
});
</textarea>

Or any time after the object has been created:

<textarea class="javascript">
w2ui.sidebar.topHTML = 'some text';
w2ui.sidebar.refresh();
</textarea>